The present invention relates to a mandrel for producing glass tubes or rods, particularly for the Vello process, the A-drawing process and preferably for the Danner process.
A common used process for manufacturing glass tubes or rods is the Vello process. The glass melt flows from an oven into a bowl in which a hollow vertical mandrel, the Vello needle, is mounted, or a bell surrounded by an orifice ring. The glass flows through the annular space between the bell and the ring and travels over a line of rollers to a drawing machine. Tubing is made by blowing air through a bell with a hollow tip and rod is produced by using a bell with a solid tip. The tube firstly flows vertically downwards, is then diverted horizontally and drawn off mechanically via a gravity-roller conveyor (as in the Danner process) cooled and cut to length.
In the A-drawing process (down-draw process), the glass melt already flows in tube form from the feeder, since it exits through a cylindrical form. It flows over the drawing tool, a mandrel, here an A-drawing needle, where the glass is formed into a tube. In this process, the operation can be performed with air. The tube flows vertically downward and is cut to length without deflection at temperatures of about 300° C.
The Danner Process was developed for the continuous production of glass tubing and rod. In the Danner process, a slightly tilted, slowly rotating tube, the Danner mandrel, on which a continuous strand of glass melt accumulates, is used as a drawing tool. At the lower end of the mandrel, the head of the mandrel, the glass is drawn off under the formation of a tube, whereby a cavity is formed by supplying air or gas through the hollow shaft of the mandrel. After deflection to the horizontal, the solidified tube passes through a gravity-roller conveyor to the drawing machine, behind which separation into tube sections is carried out by chopping. A further coating of the inner side of the tube is described for example in the U.S. Pat. No. 6,595,029, which relates to a coated mandrel and which is herewith incorporated by reference.
Since the above described processes operate at very high temperatures, the mandrel has to be heat-resistant. Furthermore, the temperature gradient between the rear end of the mandrel, particularly the Danner blowpipe, where the glass is dispensed to the mandrel and the front end where the glass is removed or pulled away is in the range of 300-400K. Typical ceramic mandrels or mandrels with a ceramic surface provide only a very short lifetime which depends on the glass type and the processing temperature. Furthermore, the use of ceramic mandrels can impure the glass tubes. Alternatives, with ceramic mandrels coated or covered with stainless steel lead to high stressing forces between the different materials, which may result in buckling or warping of the surface, especially for composite products with different thermal expansion coefficients